Integrated Gasification Combined Cycle (IGCC) technology couples a complex coal gasification process plant with a synthesis gas-fired combustion turbine combined cycle power plant. The IGCC process typically involves a two-stage combustion operation, which typically includes a cleanup between the stages. The first stage employs a gasifier where partial oxidation of the coal is carried out by limiting the oxidant supply. Other methods, such as steam reforming, may also be used to produce the synthesis gas. The thus-produced synthesis gas, a mixture mostly of CO and H2, is then typically scrubbed to remove impurities such as sulfur, and sent to a second stage. In the second stage, the synthesis gas is burned in a combustion turbine to complete the oxidation and produce energy.
To produce the synthesis gas, sources of carbon other than coal may be used. This so-called gas turbine/combined cycle (GT/CC) technology operates equally well with a variety of carbon-containing feed stocks such as liquid and solid hydrocarbons, biomass, asphalt, tires, coke residue, and the like.
Of extreme importance to an IGCC plant is the integration of the entire system—the gasification unit and the combustion turbine. Because it is impractical to store significant quantities of synthesis gas, the combustion turbine must remain operational whenever the gasification plant is in operation. Shutting down the combustion turbine typically requires an immediate shutdown of the gasification plant. It is also difficult to run the gasification plant at only part load, and hence it is necessary to run the combustion turbine in at least a base load configuration. These are significant operating limitations.
Coal-derived synthesis gas has a very low heating value (115-125 BTU/scf LHV) compared to that of natural gas (800-1000 BTU/scf LHV). Because of this, the combustion hardware on a synthesis gas-fired combustion turbine must be substantially modified from that normally used on a natural gas-fired, combustion turbine. The cost of these modifications can be significant, adding to the cost of the plant, and creating additional maintenance issues for the operator.
Rather than burning the synthesis gas for its energy value, the synthesis gas may be converted into hydrocarbons. These so-called gas-to-liquid (GTL) and coal-to-liquid (CTL) processes are well known. Several methods are available to carry out the conversion. The Fischer-Tropsch process is but one example in which CO and H2 are catalyzed into hydrocarbons. Hydrocarbons produced by the Fischer-Tropsch process include C1-C200 or higher, with most being in the range of about C1-C50.
In the past 15 years, however, liquid fuels have not been the fuels of choice for combustion turbines. This is because of the higher levels of pollution typically associated with burning liquid fuels compared to burning gaseous fuels such as natural gas. Liquid fuels are traditionally burned in non-premixed (or diffusion) mode, which leads to regions of relatively high temperature within the combustor. Since non-premixed combustion can increase the amounts of pollutants such as NOx, premixed combustors have been developed for gas turbines. These allow for greater control of the temperature field in the combustor. In addition, the practice of introducing water or steam into the combustor to reduce emissions of NOx compounds when burning liquid fuels in non-premixed mode also has a detrimental effect on the efficiency and lifetime of the combustion turbine hardware.
U.S. Pat. No. 7,089,745, the contents of which are hereby incorporated by reference, discloses a system for vaporization of liquid fuels for combustion and method of use.